As a heat exchanger such as an oil cooler, there has been known a heat exchanger in which a flow path for fluid is formed between adjacent core plates by stacking a plurality of relatively thin core plates made of an aluminium alloy, etc. In this type of heat exchanger, as disclosed in Patent Publication 1, it is general that a core portion formed of stacked core plates is brazed on a bottom plate whose plate thickness is relatively thicker than that of the core plate, and that the heat exchanger is mounted on a mating member through this bottom plate. In addition, in Patent Publication 1, it is disclosed that a configuration of a heat exchanger, in which an oil flow path and a cooling water flow path formed by a plurality of core plates are alternately disposed. Moreover, there has also been known a type of a heat exchanger in which only oil flow paths are formed by stacking a plurality of core plates, and which is accommodated in a chamber through which cooling water flows.
At the same time when brazing a plurality of core plates on each other, brazing the core plates with a bottom plate is performed by heating in a furnace in a condition in which each member has been assembled. As a supply of brazing material, there is a method that a clad material having a brazing material layer on its surface is used as a core plate and a bottom plate, or that a sheetlike brazing material is sandwiched between joint surfaces.
In Patent Publication 1, an undersurface of the core plate at the lower end (that is, a joint surface to be joined to a bottom plate) is flat. However, in case that the core plate at the lower end has some kinds of boss portions, a part that is a large gap is locally formed between the undersurface of the core plate at the lower end and a surface of the bottom plate, which become joint surfaces. Therefore, it is not preferable because a melted brazing material tends to be excessively gathered at the boss portion at the time of brazing. For example, in case that the boss portions are formed on the core plate at the lower end and on a second core plate adjacent to this so as to be joined to each other around a communication port, the boss portion of the core plate at the lower end is formed to swell upwardly so as to be away from a surface of the bottom plate. Therefore, as mentioned above, the melted brazing material tends to be gathered. In this way, if the melted brazing material of a high temperature is excessively gathered, the core plate is locally excessively heated. Therefore, to avoid thermal damage of the core plate, temperature control at the time of the brazing is extremely strictly required.